Amgen Inc., the Thousand Oaks biotechnology concern, will pay $500,000 in initial licensing fees to Progenitor Inc. in Columbus, Ohio, for rights to its leptin receptor technology. Amgen already owns other rights needed to produce leptin, a possible fat-reducing drug that Amgen now has in human clinical trials. Some analysts have doubted the potential value of Amgen's developmental drug because studies have shown that many obese people already have high levels of leptin in their bodies. However, Progenitor's technology could be a key step in developing a fat-reduction drug, said Jim McCamant, editor of Medical Technology Stock Letter in Berkeley, because a receptor triggers the biological action that accepts a drug's molecule. Amgen hopes leptin will send signals to a patient's brain that the stomach is full. Amgen is one of many drug companies trying to come up with a product that can treat obesity, a potential multibillion-dollar-a-year market. Amgen could pay Progenitor another $22 million if the licensed technology leads to a worldwide launch of the drug. Amgen's shares lost 37.5 cents to close at $56.75 on Nasdaq.